The majority of inferior mesenteric ganglion (IMG) neurons receive afferent synaptic input from a subset of enteric neurons distributed throughout the colon. This major afferent pathway targets sympathetic IMG neurons which control motility and intestinal secretion. NADPH- diaphorase and neuronal nitric oxide synthase are localized in the majority of colonofugal neurons which project to the IMG. There is currently little information on the role of NO in the IMG. Many of the actions of NO in gut and vascular smooth muscle appear to be due to elevation of cGMP through activation of NO-sensitive soluble guanylate cyclase. There are no comparable studies for IMG neurons. One objective of this proposal is to provide an understanding of how NO modulates the synaptic gain of the colon-IMG reflex, and the role of NO-generated cGMP in this reflex pathway. Heme oxygenase 2, the key enzyme for production of CO, is present in the IMG. Little is known regarding the role of CO in prevertebral ganglion function. The second objective, therefore. is to investigate the role of CO in the IMG by localizing the distribution of heme-oxygenase 2, measuring CO production by the IMG, and by evaluating the effect of CO on ganglion cell excitability and synaptic transmission. Lastly. the structural morphology of the neurons which comprise the colon-IMG reflex is not well understood. Thus, the third aim is to reconstruct in 3D using laser scanning confocal microscopy the neurons of the colon-IMG reflex. the surface distribution of putative synaptic regions. and the distribution of specific receptor clusters known to regulate the reflex. The methods used include intracellular microelectrode recordings, immunohistochemical staining, radioimmunoassay, radiothin layer chromatography, and confocal laser scanning microscopy. This work is expected to augment knowledge of the circuits of the peripheral nervous system which modulate fluid loading and motility of the colon.